Phase modulation system



Filed July l2, 1945 ast/M me \l|\ a M TM f M w, j A i N d@ g M l: wg wc w/W, (i f m1.. /f. w/A Hf) UIIIIJ r j ,QM/Mm?. W. a y, M Z m 7 f W CMM w m 1 Patented Sept. 27, 1949 PHASE MODULATION SYSTEM Bernard J. Cosman, Arlington, Mass., assignor to` Harvey Radio Laboratories, Inc., Cambridge, Mass., a corporation of Massachusetts Application July 12, 1945, Serial No. 604,544

(Cl. S32-24) 10 Claims.

The present invention relates to a system of transmitting signals by modulating a high frequency carrier wave in accordance with signal impulses, and more particularly to angular modulation through. varying the instantaneous phase angle of the carrier Wave,v this variation being proportionate to the instantaneous value of the modulating signal impulse;

lt hasheretofore been. proposed to modulate the phase of a carrier wave train by modulating the amplitude of one' orl both. of two components thereof, these components' having the same frequency but being so displaced as' to phase. that they always subtend a phase angle of 90 when the phase is not shifted .onesense or the other. Either system involves balanced operation of pairs of tubes as elements of a balanced amplitude modulation circuit or of a symmetric audio imput network. The first of the above mentioned systems, namely that with a single amplitude modulated component, uses. balanced amplitude modulation of that component, whereas the other system employs amplitude modulation of both components; both require balanced audio imput. In systems of this type, any unbal'ance in the above mentioned circuit elements gives rise to asymetric phase modulation, and since it is practically impossible toV prevent such unbalance, the phase swing hasto be limited to very small angles, unless considerable distortion is tolerated. Further, each of theseY systems calls, in addition to twoor more tubes working in balance, for a device providing fairly exact 90 phase shift, which requires comparatively complex circuits which are not absolutely reliable.

It is the principal object of the present invention to provide a method of phase modulation and apparatus for carrying .out that method, wherein one of two out-o-f-phase carrier components of the same frequency is amplitude modulated, but which lsindependent of balanced operation and does not require complex and exactly operating phase' shifting devices', and which is, in spite of extreme simplicity, independent .of variations of circuit components and permits easy monitoring of the conditions for maintaining symmetric phase swing.

In one of itsaspects, the invention contemplates the vectorial summation of a single-ended amplitude modulated Vector with an unmodulated vector of the same frequency at'such phase angle that the zero phase modulated resultant and the amplitude modulated component are normal to each other. At the same time, the phase angle of the two components, namely the immodulated component and the amplitude modulated component, has such a value that the resultant vector remains fully phase modulated at the lowest amplitude of the single ended amplitude modulated component vector.` In another aspect, the `in vention provides a vphase modulation system wherein unmodul-atedand amplitude modulated component vectors form a reex angle (that is an angle greater4 than 189), with the zero modulation vector of the resultant, phase modulated wave being at right angles to the amplitude modulatedv component vector. In a further aspect, the invention utilizes twov carrier components at a phase angle of approximately 45, one of the components bei-ng ampliedand in that 'stage .of the transmission shifted ra further 180 so that the two components are combined-.while forming an angle of approximately 225, with the zero phase modulating amplitude of the amplitude modulated Vcomponent providing a resultant vector which is normal to the amplitude modulated vector.

Thesecharacteristics permit, as an importantv feature of the invention, the use lof circuit elementswhich combine simple construction with exact functioning, namely a network for shifting a vector- 45, and an amplification and modula-Vv tion arrangement which inherently carries out a 180 phase shiftbetween input and output waves.

In still another aspect .of the invention, a phase modulated resultant wave is obtained by combining two phase'displaced vectors of the same frequency, one of which is single-end amplitude modulated, such that the phase displacement ine cludes substantially 45 introduced by a phase shifting network, and a further shift of 130 introduced-by the function of an electronic tube.-

The invention thus provides* a phase modulation system with a minimum number. of the simplest:

possible parts, and which isinherently insensitive to incidental changes in element characteristics.

An additional feature, of. the yinvention is the possibility of maintaining symmetric phase swingv by monitoring the output o f theamplitude moduphase modulation niethod'according to the in-Y vention; I y *I Fig. 2 is a vector diagram further explaining the method carried out with circuits according te Fig. 1; and

Fig. 3 is a detailed circuit diagram of a transmitter incorporating the invention.

The principle of the phase modulation system according to the invention will first be generally explained with reference to Figs. l and 2.

In the block diagram of Fig. 1, numeral I I indicates an oscillator which furnishes the carrier voltage Ec, vectorially indicated in Fig. 2. The carrier voltage Ec is directly applied, if desired through a buffer network I6, to the output utilizing instrumentality, herein indicated by transmitter I9. The voltage Ec is also applied to a network I4 which shifts its phase an acute angle, for example 45 and applies the phase shifted voltage Ep to an unbalanced amplitude modulator device I5. In this device I5, voltage Ep isv by way of amplification shifted a further 189 and also amplitude modulated, under the control of voltage Ea supplied by an unbalancedaudio signal imput device indicated at I2.

Fig.- 2 indicates the phase relation of voltages Ec and Ep. As likewise indicated in Fig. 2, the amplii'led and modulated voltage Ep appears as modulated voltage Em which is shifted 180 relatively to Ep and varies, as controlled by the signal voltage Ed, between values Eml and Emh. Voltage Em is then combined, if desirable through a buffer circuit I1, with the original carrier voltage Ec, these two components furnishing the resultant voltage Eo. As will be evident from Fig. 2, the vector Eo which corresponds to the intermediate or unmodulated value of Em forms an angle of 90 with the vectorial direction of voltage Ep and Em, the locus of the phase modulated voltage vectors Eo being a line a parallel to the direction of Ep and Em. As clearly indicated in Fig. 2, the total phase shift is an acute angle from Ec to Ep, and 180`from Ep to Em, so that Em and Ep form a reflex angle.

In this system, the criterion for symmetrical phase modulation is that Eo be normal to the originally phase shifted and amplitude modulated values Ep, Em. This condition for maintaining symmetrical phase swing 1M, as indicated in Fig. 2, can be readily maintained by monitoring the voltage Em at its proper value. It will now be evident that the present system requires only a phase shifting circuit which is of the condenserresistor type wherein wRC=1, instead of the complicated and critical L.C. circuits which are required for providing the 90 shift upon which previously suggested phase modulation systems are based. The amplitude modulator is single-ended, therefore requiring only one tube instead of the two tubes required in previously suggested systems which are based on balanced modulation. Similarly, the audio frequency input effecting the modulation of vector Em, may be unbalanced in the present system instead of the balanced type required in previous circuits.

A practical phase modulation circuit according to the present invention will now be described with reference to Fig. 3.

InV Fig. 3, numeral II again indicates an oscillator circuit furnishing the carrier voltage Ec which is supplied to the output device, herein indicated as transmitter I9, if desirable through a buier herein indicated as resistor R9.

The oscillator I I also feeds into a phase shifting network N consisting of variable capacitor C and resistor R, connected between wire 2I and wire 22, wire 22 representing the negative side of the energy input which is indicated at 20 and whose positive side is represented by wire 23.

At adjustable tap n of network Voltage Ep is derived and applied, through a blocking capacitor C2, to grid gI of a suitable multi-grid tube V, for example of type 6SA7.

The second control grid g2 of tube V is connected to the audio input circuit I2, through wire 24. Grids gi and g2 are connected to wire 22 through grid resistors RI and R2 respectively. The cathode lc of tube V is connected to wire 22 through the usual bias resistor R4 which is bridged by a bypass capacitor C4. The plate a of tube V Y is supplied from wire 23 through a filter network consisting of adjustable capacitor C5 and inductor L5, which may also be adjustable. The ampliiiedand phase modulated voltage Em is fed, if desired through a buffer, herein indicated as buier resistor R9, into wire 2I and transmitter I9.

The suppressor t is as usual connected to cathode lc, and the screens s are connected to wire 23 and bypassed by capacitor C1.

When buier anipliers are used instead of resistors R8, R9, the phase shift characteristics of these buffers must be substantially identical, so that the phase relation of the combining vectors is not altered.

For the purpose of monitoring the phase relation of unmodulated and amplitude modulated components, respectively, substantially identical vacuum tube voltmeter circuits 3l and 32 are connected to wire 2 I', carrying unmodulated wave train Ec, and the output circuit of tube V, carrying amplitude modulated wave train Em. These voltmeter circuits are of the conventional type suited for measuringthe linear average voltage,

, and are counter-connected into a zero reading instrument 33 of any type suitable for indicating the difference between the voltage values furnished bycircuits3land32. ff Y vRecapitulating the` above explanation ofthe operation of a phase modulation system according -to the present invention with reference to Fig. 3, the circuit accordingto that figure functions as follows:

Oscillator II feeds voltage Ec through buffer resistor R8 into transmitterr I9. .The approximately 45 phase shifted voltage'Ep,` derived at tap n of phase shifting network N Vis applied through blocking capacitor :C2 to grid gI, plate a thus furnishing vector Em, compare Fig. 2. The audio frequency supplied bynetwork I2 is applied throughwire 24 to grid g2, and effects the amplitude modulation ofv vector Em,.between the values of Emh and Eml.

The amplitude modulated voltage Em is fed, if desired through buierresistor R9 into wire 2I, where it is combined with the carrier Voltage Ec to provide the resultant phase modulated voltage Eo, which varies through angles iM. The transmitter I9 is thus furnished with a phase modulated input wave .train which, if desired. may rst be passed through a-buier amplifier, and thereupon further utilized, for example for wireless broadcasting purposes. f

If the phase difference between Ec and Ep is kept at substantially 45, which can be easily accomplished by adjusting capacitor C and tap n of resistor R, the condition of perpendicularity between vectors Eo and Em can be maintained by monitoring `these voltages for zero reading at indicator 33. For phase diierences other than 45, the indicator has to be calibrated for zero reading at symmetrical phase modulation, as detected from the phase modulated output.

The dimensionsv and characteristics of the various circuit elements are. selected according to conventional rules, depending upon the energy rating of' the device and the tolerated deviations from the theoretically determined values as indicated in 2:, care being taken that the impedance looking intok thev -modulator circuit: is fairly high as compared to the input impedance.

It will non be evident that the above mentionedv value of 45 for the phase shift between vectors Ec and Ep is not and that the present sys-f tem will operate satisfactorily, so long as the locus a of the ends of vectors En is parallel to the direction of the vector'Em, and so long as the mini-- mum value Eml of mcdulated'vector Em does not aitogether disappear', that is, so long as angle -Mis not greater than the angle between Eo and Ec. Y

It will fmther' be evident, that the circuit elements shown in Fig. 3 need not necessarily be used, but that any combination of elements performing the function indicated in Fig. Z may be employed,v so long as they conform to the -basic concept of the invention, namelyv the combination of two voltage vectors at an angle of substantiaily more than 180, one of these vectors being single-end amplitude modulated, and the resultant unmodulated vector being normal to the amplitude modulated component vector.

I claim:

l. In the art of signal transmission by modulated high frequency Waves, the method of phase modulating a carrier Wavel train under control of signal impulses, which method comprises the steps of generating a wave train, deriving'. from said wave train a second Wave train of substantially equal frequency but hai/ing a phase which differs from that of saidrst train by an angle greater than 180 but less than-276, modulating the amplitude of said second train under control of signal impulses while maintaining it on one side of its Zero value and While maintaining said frequency, and vectorially adding said second train to the unmodified rst train to form a phase modulated resultant Wave, the phase and amplitude values of said two tra-ins being so selected that said phase modulation of said resultant Wave remains substantially symmetrical.

2. In the art of signal transmission by angularly modulated high frequency waves, the method of phase modulating a carrier Wave train under control of signal impulses, which method comprises the steps of generating a wave train of substantially constant frequency, deriving from said wave train a second Wave train of substantially the same frequency but having a phase displacement of more than 180 but less than 270 relatively thereto, unidirectionally modulating the amplitude of said second train proportionate to a signal While maintaining its polarity and frequency, and vectorially adding said second train to the unmodified first train to form a phase modulated resultant wave, the values of said two trains being so selected that the direction of the unmodulated vector of said resultant Wave is substantially normal to the direction of the vector of said second train.

3. In the art of signal transmission by modulated high frequency Waves, the method of phase modulating a carrier Wave train under control of signal impulses, Which method comprises the steps of generating a wave train of substantially constant frequency, deriving from said Wave train a second Wave train of substantially the same frequency but having a phase displacement of appronimately 45 relatively thereto, amplifying' said second Wave train While shiftingyits phase a further 186x, to yobtain a third wave train at' approximately 225' to said first Wave train, unidirectionally modulating the amplitude of` said third train proportionate to said signal impulses While maintaining said frequency, and vectorially adding said unmodified rst trainand said third train to form a phase modulated resultant/wave, the values of said, trains being so selected that the direction of the vector of the unmoduiated Wave is substantially normal to the direction of said third train.

4..In the art of` signal transmission by modulated high frequency Waves, the method of phase modulating av carrier Wave train under control of signal impulses, which method vcomprises the steps ofr generating a, Wave train of substantialiy constant frequency, deriving. from said wave train a second wave train of substantially the same frequency buthaving an acute angle phase displacenient relatively thereto, amplifying said second Wave trainfwhile substantially reversing its phase to obtain a third Wave train at a. phase angle relatively to said first. train which is greater than 180 but less thanZ''iY, unidirectionally modulating the amplitude of said third train proportionate to said signal impulses' maintaining said frequency, vectorialiy adding said unmodied first train and said third train to forni a phase modulated' resultant Wave, and regulating the values Gf said trains such that the direction of the vector of the nnmodulated resultant wave is substantial'fiy normal tothe direction of said third train. I 5. In the art of signal transmission by modulated high frequency Waves, apparatus for phase modulatingra carrier Wave train under control of signal impulses,v comprising means for generating a wave train, means for deriving from wavetrain a second Wave train Vof substantialiy equal frequency but` having a phase which differs from that of said first train by an angle greater than 180 but less than 270, means for unidirectionally modulating the amplitude of said second train under control of said signal impulses While maintaining said frequency, and means for vectorially adding said second train to the unmodified first train to form a phase modulated resultant wave Whose unmodulated vector is substantially normal to the vector of said second train.

6. In the art of signal transmission by modulated high frequency Waves, apparatus for phase modulating a carrier Wave train under control of signal impulses, comprising means supplying a signal wave to be transmitted, means for generating a Wave train, means for deriving fromk said wave train a second wave train of substantially the same frequency but having phase displacement greater than 180 "but less than 270 relatively thereto, means for unidirectionally modulating the amplitude of said second train proportionate to said signal wave while maintaining said frequency, means for vectorially adding said unmodified rst train and said second train to form a phase modulated resultant wave, and means for maintaining the unmodulated vector of said resultant wave substantially normal to the vector of said second train.

7. In the art of signal transmission by modulated high frequency waves, apparatus for phase modulating a carrier Wave train under control of signal impulses, comprising means for generating a wave train of substantially constant frequency, means for deriving from said wave train a second wave train of substantially the same frequency but having a phase displacement greater than 180 but less than 270 relativelylthereto, means for unidirectionally modulating the amplitude of said second train proportionate to said signal impulses while maintaining said frequency, and means for vectorially adding said unmodified rst train and said third train to form a phase modulated resultant wave whose unmodula-ted vector is substantially normal to the vector of said second train.

8. In the art of signal transmission by m'odulated high frequency waves, apparatus for phase modulating a carrier wave train under control of signal impulses, comprising means for generating a wave train, means for deriving from said wave train a second wave train of substantially the same frequency but having an acute angle phase displacement relatively thereto, means for amplifying said second wave train while substantially reversing its phase to obtain.

a third wave train, means for unidirectionally modulating the amplitude of said third train under control of said signal impulses while maintaining said frequency, means for vectorially adding said unmodified rst train and said third train to form a phase modulated resultant wave, and means for maintaining the unmodulated vector of said resultant wave substantially normal to the vector of said third train.

9. Phase modulation apparatus comprising a transmission line Ywith two conductors, an oscillator of substantially xed frequency feeding into said line, a phase shifting network including a capacitor and a resistor connected in series across said rst and said second conductors, a high vacuum tube having anode, cathode and two control electrodes, a connection from a tap between said capacitor and said resistor to one of said electrodes, signal impulse supply means having two terminals, one of which is connected to said second conductor, a connection from said second electrode'to the other terminal of said sig-r a substantially purely ohmic resistance connection from said transmitter through said rst conductor to said oscillator.

10. Phase modulation apparatus comprising a transmission line with two conductors, an oscillator of substantially xed frequency feeding into said line, a phase shifting network including a capacitor and a resistor connected in series across said rst and said second conductors, a high vacuum tube having anode, cathode and two control electrodesa connection from a tap between said capacitor and said resistor to one of said electrodes, signal impulse supply means having two terminals, one of which is connected to said second conductor, a connection from said second electrode'to the other terminal of said signal supply means, means for connecting said anode to said rst conductor, means for adjusting the amplitude of the output current of said tube, a transmitter, and a substantially purely ohmic resistance connection from said transmitter through said iirst conductor to said oscillator.

BERNARD J. COSMAN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,687,225 Peterson Oct. 9, 1928 1,892,383 Chiveix Dec. 27, 1932 1,964,522 Lewis June 26, 1934 1,994,048 Runington Mar. 12, 1935 2,027,044 Hofer et a1. Jan. 7, 1936 2,238,249 Crosby Apr. 15, 1941 

